MP4078

– PSR OFFLINE CONTROLLER WITH PFC

MP4078 Rev. 1.0

www.MonolithicPower.com

21

7/21/2020

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2020 MPS. All Rights Reserved.

Where BMAX is the maximum allowable flux

density (T) (typically 0.2T to 0.3T for common

ferrite cores), KU is the window factor of the

transformer (typically 0.15 to 0.3), and J is the

current density coefficient (typically 4A/mm2 to

6A/mm2).

Choose a ferrite core with an effective area

product (APREAL) that exceeds APMAX. Calculate

the turns on the primary winding (NP) and

secondary winding (NS) with Equation (12) and

Equation (13), respectively:







M

PRI _ PK _ MAX

P

MAX

E

LI

N

BA

(12)

 P

S

N

N

N

(13)

Where AE is the effective core area.

The auxiliary winding provides the power supply

for the MP4078 through a diode on the VCC pin

during normal operation. The auxiliary winding

(NAUX) can be estimated with Equation (14):







CC _ NOM

DIODE _ AUX

AUX

S

O

DIODE

VV

NN

VV

(14)

Where VCC_NOM is the voltage on the VCC pin

during normal operation, and VDIODE_AUX is the

forward voltage of VCC diode.

Ensure

the

inductance

leakage

on

the

transformer is sufficiently low. This helps to

decrease the switching spike for ZCD sensing

and EMI performance.

Selecting the External Pull-Up Resistor on

MULT

The peak voltage on the MULT pin (VMULT) is

directly related to the on time, and is determined

by the external pull-up resistor on the MULT pin

(RMULT_UP) and the internal pull-down resistor

(RMULT).

In most cases, it is recommended to match the

output of the error amplifier (VCOMP) with the

VCOMP_FULL value under full-load conditions. The

corresponding on time is the turn-on time during

full-load conditions (tON_FULL) (see Figure 11).

COMP

VCOMP_FULL

tON

VCOMP_HALF

VCOMPH

tON_FULL

Figure 11: tON vs VCOMP

RMULT_UP can be estimated with Equation (15):







 

 







IN _ RMS _ MIN

ON _ MAX

MULT _ UP

MULT

SLEW

COMP _ FULL

2

V

t

R

R

1

KV

(15)

Selecting the Current-Sense Resistor

A proper current-sense resistor both ensures

normal operation across the output load range

and implements cycle-by-cycle current limitation

to guarantee circuit reliability in the event of an

overload condition.

The current-sense resistor (RSENSE) can be

calculated with Equation (16):





CS _ MAX

SENSE

OL

PRI _ PK _ MAX

V

R

I

(16)

Where VCS_MAX is the maximum current limit

(typically 880mV), and

αOL is the coefficient of the

peak primary-side current during overload (1.3 to

1.5 is typically recommended).

Selecting the External Primary MOSFET

The drain-source voltage rating and current

rating are two dominant factors to consider when

choosing the external primary MOSFET. The

maximum drain-to-source voltage (VDS_EXT_MAX)

appears

at

the

high-line

input

voltage

(VIN_RMS_MAX). VDS_EXT_MAX can be calculated with

Equation (17):





 



 

DS _ EXT _ MAX

IN _ RMS _ MAX

O

DIODE

PRI _ SPIKE

V

2 V

N (V

V

)

V

(17)

Where

∆VPRI_SPIKE is the turn-off spike on an

external MOSFET. This spike is caused by

parasitic parameters and the

transformer’s

leakage inductance. With proper transformer

and primary snubber design, this value should

not exceed 100V in normal operation for

universal input applications.